1. Field of the Invention
The present invention relates to a laundry apparatus employing an inverter system in which a DC brushless motor drives a pulsator.
2. Description of the Prior Art
To perform effective and reliable washing, it is essential that the water amount, the water flow rate and the washing duration are set in accordance with the laundry amount.
Several detection methods have been proposed for laundry machines in which induction motors are utilized to drive pulsators. However, the rotating speed of induction motors is not adjustable. On the contrary, there are advantageous features in a washing machine having a DC brushless motor for driving the pulsator, such as an improved washing efficiency in that the rotating speed of the DC brushless motor is freely adjustable through an inverter circuit and noises produced from the motor become less the motor is of a brushless type.
Referring to FIG. 1, a conventional method for detecting the laundry amount is explained as follows.
Torque T is expressed by the following equation. EQU T=C.sub.1 .multidot.i.multidot.B.multidot.l (1)
where,
C.sub.1 : proportional constant, PA1 i: motor current, PA1 B: flux density, PA1 l: conductor effective length. PA1 C.sub.2 : proportional constant, PA1 E: induction voltage PA1 a DC brushless motor to drive a pulsator; PA1 an inverter circuit having outputs which drive the DC brushless motor by means of periodically switching an input DC voltage with a switching device; PA1 an inverter control circuit which controls the switching IC operation of the inverter circuit to adjust the output of the inverter circuit; and PA1 laundry detecting means for detecting the laundry amount on the basis of the correlation between the current flowing through the inverter circuit under the constant control state of the switching IC and the rotating speed of the DC brushless motor.
Rotating speed N (rpm) is given by EQU N=C.sub.2 .multidot.E/B.multidot.l (2)
where,
FIG. 1 shows the characteristics of rotating speed N over torque T (T-N characteristic) and average inverter current I.sub.DC over torque T (T-I.sub.DC characteristic) when the on-off duty ratio of transistors in an inverter circuit is 50%. The broken lines in FIG. 1 show the load lines for the laundry amount of 500 g, 2 kg and 5 kg, respectively.
In the conventional way of detecting the laundry amount, its amount is measured by the change in the average inverter current proportional to the torque, based on equation (1).
It is a well-known fact that the commercial power supply voltages fluctuate within the range of 10%, namely from 90 V to 110 V against the fixed 100 V. When this fluctuation occurs to the conventional washing machine, it will be difficult for the same laundry amount to be properly sensed since such a fluctuation in the commercial power supply voltage causes the average inverter current value to change as well. That is to say, when the commercial power supply voltage fluctuates in the usual manner, the different inverter current value is incorrectly detected for the laundry amount in question. Thus the motor rotates with a rotating speed based on the incorrect average inverter current value detected.
The laundry amount can also be detected by realizing a change in the rotating speed using equation (2). However, this method is also not reliable since it becomes hard to detect the laundry amount when the commercial power supply voltages fluctuate to cause the rotating speed to vary.